The fabrication of a single semiconductor device can require the careful synchronization and precisely measured delivery of as many as a dozen gases to a process chamber. Various recipes are used in the fabrication process, and many discrete processing steps where a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, etc., may be required. The steps used, their particular sequence and the materials involved all contribute to the making of particular devices.
Accordingly, wafer fabrication facilities are commonly organized to include areas in which chemical vapor deposition, plasma deposition, plasma etching, sputtering and other similar gas manufacturing processes are carried out. The processing tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition, must be supplied with various process gases. Pure gases must be supplied to the tools in contaminant-free, precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks, which are connected via piping or conduit to a gas box. The gas box delivers contaminant-free, precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a process tool. The gas box, or gas metering system includes a plurality of gas paths having gas metering units, such as valves, pressure regulators and transducers, mass flow controllers and filters/purifiers. Each gas path has its own inlet for connection to separate sources of gas, but all of the gas paths converge into a single outlet, such as a gas manifold, for connection to the process tool.
It is desirable and often times necessary when delivering a gas in measured amounts to be able to obtain accurate measurements of gas flow conditions in gas delivery systems used to deliver such a gas, as for example, when testing the accuracy of or calibrating a mass flow controller. One way to accomplish this is by measuring the rate of change in temperature and pressure of a gas in a chamber of known volume and calculating mass flow from the measured values. According to this so-called “rate of pressure rise”, or “rate-of-rise” (ROR) method, a gas flow is conducted through a device-under-test (DUT), such as a mass flow meter, into an evacuated, volume-calibrated chamber for a measured interval of time, Δt. The changes in pressure (ΔP) and temperature (ΔT) of the gas in the chamber are measured and corrected to a standard temperature (T0) and pressure (P0). The term “standard”, as used herein, means “standard conditions”, usually defined as an “absolute” temperature of 273.15K and an “absolute” pressure of 1 atmosphere. The gas flow rate can then be calculated from the change in pressure over time (ΔP/Δt) and the change in temperature over time (ΔT/Δt) in the known volume.
Many gas delivery systems employ gases which behave substantially as ideal gases. In other words, their behavior can be accurately predicted by and modeled in accordance with the ideal gas law, expressed as PV=nRT, where P is pressure, V is volume, n is the number of moles of the gas, R is the universal gas constant, and T is absolute temperature. The relationship between pressure change and temperature change of a substantially ideal gas in a fixed volume over time is constant regardless of the gas. Thus, the ideal gas law relationship can be used to determine n, the mass, i.e., the number of moles of gas in the chamber. In those situations in which gases behave differently from an ideal gas, correction functions can be used to render the measurement of pressure and temperature changes over time more accurate.
Some devices use the ROR method to verify flow. For example, U.S. Pat. No. 5,684,245 to Hinkle, which is assigned to the assignee of the present invention, discloses and claims an apparatus for and method of measuring mass flow of a gas in a gas delivery system using the ROR method. The assignee of the present invention, MKS Instruments Inc. of Andover, Mass., also provides ROR flow verifier products such as the Tru-Flo™ In-Situ Mass Flow Verifier, and the GBROR™ (gas box rate-of-rise) In-Situ Mass Flow Verifier.
What is still desired is a new and improved system and method for verifying and calibrating flow control devices in a gas metering system. Preferably, the new and improved system and method will employ a ROR flow verifier. In addition, the new and improved system and method will preferably provide in-situ verification and calibration of flow control devices, so that the verification and calibration does not require removal of the flow control devices from the gas metering system.